Liquid crystal display devices are used nowadays in numerous applications such as clocks, household appliances, electronic calculators, audio equipment, etc. There is a growing tendency to replace cathode ray tubes by liquid crystal display devices being favoured for their smaller volume and lower power consumption. In some applications like e.g. laptop computers and pocket TV's liquid crystal display devices are even without competition.
High definition television in its ultimate version will require screen diagonals exceeding 50 inch (see P. Plezhko in the periodical Information Display September 1991, Vol. 7 no. 9, p. 19 a.f.). Although not yet in existence CRT-based 50 inch screens can be expected to be very impractical because of their weight and size. Liquid crystal technology is basically able to produce high definition television (HDTV) screens with moderate weight and size.
Liquid crystal display devices generally include two spaced glass panels, which define a sealed cavity, which is filled with a liquid crystal material. The glass plates are covered with a transparent electrode layer which may be patterned in such a way that a mosaic of picture elements (pixels) is created.
Full color reproduction is made possible by the use of a color filter array element inside the liquid crystal display device.
Two addressing systems are used to drive the display : either a passive system or an active system.
According to the passive system in the liquid crystal device the 2electrode layers are patterned in a regular array of stripes. The stripes on one plate are perpendicular to those on the other plate.
The application of a voltage across two opposing stripes causes a change in the optical properties of the liquid crystal material situated at the crossing point of the two stripes, resulting in a change of the light transmission through the energized picture element called pixel.
According to the active system, which greatly improves the performance of the liquid crystal display device, each pixel has its own individual microelectronic switch, which means that such a microswitch is connected to an individual transparent pixel electrode, the planar size of which defines the size of the pixel. The microswitches are individually addressable and are three-terminal or two-terminal switching elements.
Three-terminal switches are formed by thin film transistors (TFT). These transistors are arrayed in a matrix pattern on a glass plate which together with a glass plate carrying a transparent uniform (non-patterned) electrode layer forms a gap filled with the liquid crystal material.
With a diode or a similar two-terminal switching device the transparent electrode layer must be patterned.
To impart color reproduction capability to the liquid crystal display device a color filter array element is provided on one of the two glass plates. In an active matrix display, examples of which are described in U.S. Pat. Nos. 5,081,004 and 5,003,302, this is usually the glass plate opposite the glass plate carrying the switching elements.
A color filter array for full color reproduction consists of red, green and blue patches arranged in a given order. For contrast improvement the color patches may be separated by a black contour line pattern delineating the individual color pixels (ref. e.g. U.S. Pat. No. 4,987,043).
In order to prevent loss of effective voltage over the liquid crystal material the color filter is preferably kept out of the electrical circuit which means that the transparent electrode is deposited on top of the color filter array element.
Several techniques for making color filter array elements have been described in the prior art.
A first widely used technique operates according to the principles of photolithography (ref. e.g. published EP-A 0 138 459) and is based on photohardening of polymers e.g. gelatin. Dichromated gelatin, doped with a photosensitizer is coated on glass, exposed through a mask, developed to harden the gelatin in the exposed areas and washed to remove the unexposed gelatin. The remaining gelatin is dyed in one of the desired colors. A new gelatin layer is coated on the dyed relief image, exposed, developed, washed and dyed in the next color, and so on. By that wash-off and dying technique 4 complete operation cycles are needed to obtain a red, green and blue color filter array having the color patches delineated with a black contour line. As an alternative dyeable or colored photopolymers are used for producing superposed colored photoresists. In the repeated exposures a great registration accuracy is required in order to obtain color filter patches matching the pixel-electrodes.
In a modified embodiment of said photoresist technique organic dyes or pigments are applied by evaporation under reduced pressure (vacuum evaporation) to form a colored pattern in correspondence with photoresist openings [ref. Proceedings of the SID, vol. 25/4, p. 281-285, (1984)]. As an alternative a mechanical precision stencil screen has been used for patternwise deposition by evaporation of dyes onto a selected substrate (ref. e.g. Japan Display 86, p. 320-322.
According to a second technique dyes are electrodeposited on patterned transparent electrodes from a dispersion of curable binder polymers, dispersing agents and colored pigments. For each color a separate deposition and curing step is needed.
According to a third technique said red, green and blue dyes are deposited by thermal transfer from a dye donor element to a dye-receiving element, comprising a transparent support, e.g. glass plate, having thereon a dye-receiving layer. Image-wise heating is preferably done by means of a laser or a high intensity light flash. For each color a separate dye transfer step must be carried out.
According to a fourth technique as described e.g. in U.S. Pat. No. 4,271,246 a method of producing a multicolor optical filter comprises the steps of (1) exposing a photographic material comprising a support and a single, i.e. one, black-and-white silver halide emulsion layer to light through a first pattern; (2) developing the exposed emulsion layer with a first coupler-containing color developer to form a pattern of a first dye; then (3) exposing an unexposed portion of said emulsion layer to light through a second pattern; (4) developing the exposed area with a second coupler-containing color developer to form a pattern of a second dye; (5) repeating exposure and development to form patterns containing dyes of third and optionally subsequent colors, thereby to form color patterns of at least two colors; and subjecting the product to a silver removal treatment after the final color development step.
All the above described techniques have in common that they require at least three (four if the black contour pattern requires a separate step) treatment steps, and some of them require very costly exposure apparatuses to reach the desired level of registration.
By the large number of production steps and the required accuracy the manufacturing yields, i.e. the percentage of the color filter array elements made in the factory which meet quality control standards are exceptionally low.
The very costly investments could be brought down when the filter production could be simplified and yet high quality maintained.
When using a multilayer color photographic silver halide material for multicolor filter production comparable to color print film used in the motion picture film industry the above mentioned problems related to image registration and large number of processing steps can be avoided. From one color negative an unlimited number of color positives on film can be produced at a very high rate. Only one exposure for each positive is needed. A great number of exposed positives can be chemically treated at the same time in the same machine. This makes the whole process very attractive from the viewpoint of yield and investment. Such process operating with a negative color image as original to form a complementary color pattern on a glass substrate has been described already in published Japanese patent application (Kokai) 60-133427.
Published European patent application 0 396 824 relates to a process for the production of a multicolor liquid crystal display device comprising a liquid crystal layer essentially consisting of nematic crystals in twisted or supertwisted configuration or smectic C (chiral smectic) ferroelectric liquid crystals wherein the liquid crystal molecules are aligned in such a way that said layer shows an electrically controllable rotation of the polarization plane of the light incident on the display. Said liquid crystal layer together with a multicolor filter element is arranged between front and rear transparent electrodes for altering pixelwise the electric field over the liquid crystal layer and said electrodes are associated respectively with a front and rear light polarizer element. Said process comprises in consecutive order the steps of:
(1) providing a photographic print material that contains on a glass support a plurality of differently spectrally sensitive silver halide emulsion layers, PA1 (2) subjecting said print material to a single step multicolor pixelwise exposure, PA1 (3) color processing said exposed print material producing thereby in each silver halide emulsion layer a differently colored pixel pattern, PA1 (4) coating said color processed print material at its silver halide emulsion layer assemblage side with a hydrophobic water-impermeable organic resin layer, and PA1 (5) depositing by vacuum-coating one of said electrodes on said organic resin layer serving as a covering layer for said silver halide emulsion layer assemblage. PA1 (A) exposing a photographic silver halide print material in order to obtain therein after color development according to the principles of subtractive color formation a color matrix (mosaic) containing blue, green and red patches or stripes, optionally separated by black contour lines, said photographic print material comprising a glass support carrying on one of its sides an assemblage of waterpermeable hydrophilic colloid layers comprising: (i) a silver halide emulsion layer being sensitive to blue light and containing a yellow dye forming color coupler, (ii) a silver halide emulsion layer sensitive to green light and containing a magenta dye forming color coupler, (iii) a silver halide emulsion layer sensitive to red light and containing a cyan dye forming color coupler, wherein in each silver halide emulsion layer the equivalent ratio of silver halide to color coupler is at least 1; PA1 (B) subjecting the thus-exposed silver halide emulsion layer assemblage to photographic color-development to create in said blue sensitive emulsion layer yellow pixels, in the green-sensitive emulsion layer magenta pixels, and in the red-sensitive emulsion layer cyan pixels, which pixels in superposition according to the principles of subtractive color photography yield a color matrix (mosaic) of blue, green and red pixels, optionally separated by black contour lines, and PA1 (C) removing from said layer assemblage any residual silver halide and image silver by bleaching and fixing followed by rinsing and drying; and PA1 (D) before introducing said multicolor filter array element in the liquid crystal device the uppermost emulsion layer of the thus processed photographic print material is coated with an organic resin to form a waterimpermeable covering layer, which covering layer for curing purposes is thermally treated at a temperature in the range of 50.degree. to 200.degree. C., and PA1 (E) on top of the thus treated resin layer a transparent electrode layer is formed, whereupon said electrode layer is coated with a resinous alignment layer, and PA1 (F) a light polarizer element is disposed on each exterior side of said glass plates.
So, before introducing said multicolor filter in the liquid crystal device the uppermost emulsion layer of the thus processed photographic print material is coated with a hydrophobic water-impermeable organic resin to form a covering layer of said resin thereon, and by vacuum-deposition on top of the thus-applied resin coating a transparent electrically conducting (electrode) layer is formed.
Said resin layer on top of the color filter array provides a good planarity and prevents the release of volatile substances from the emulsion layer during vacuum-deposition, e.g. by sputtering, of the transparent conducting layer. Usually a bake at 150.degree. C. or even higher is needed to impart by curing a good impermeability to the resin layer.
In liquid crystal displays of the so-called twisted nematic (TN) type (as are the majority of active matrix liquid crystal displays) the transparent uniformly applied electrode and also the patterned electrode are covered with an alignment layer. This layer usually consists of a heat-cured polyimide resin. Rubbing this cured layer with e.g. a nylon cloth (ref. e.g. GB-P 1,505,192) in a given direction causes an orientation of the liquid crystal molecules near the surface of the layer in the rubbing direction.
From the preceding it is clear that the multicolor filter array element is subjected to rather severe heat treatment steps during the manufacture of the liquid crystal display element. These heating steps should not give rise to discoloration of the filter and dye fading.
Most dyes formed by a reaction based on the coupling of color formers with oxidized color developer of the p-phenylenediamine type have rather limited resistance to high temperatures and tend to become yellowish or brownish, while the blues turn to dark grey.
It has been established experimentally by us that thermal degradation of color filters made by means of a multilayer color photographic silver halide material incorporating color couplers is attributed to two simultaneously occurring phenomena, i.e. breakdown of one or more of the composing dyes and coloration of the residual normally colorless color couplers still present in the processed layers.
The major contribution to coloration (yellowish or brownish) of color filters prepared by silver halide color photography based on color coupling comes from the magenta-forming color couplers of the pyrazolone type, which is representative of nearly all of the magenta color couplers used in modern color photographic materials. Furthermore said color couplers can react with magenta dyestuffs derived from them thereby causing loss of magenta dye. (P. W. Vittum and F. C. Duennebier, J. Am. Chem. Soc., 72, 1536 (1950)) Apart from this particular phenomenon the break-down of dyes is primarily determined by their structure.
It is generally known that from the 3 dyestuff types (yellow, magenta and cyan) produced on color coupling with p-phenylene diamine type developers the cyan dyes are the most susceptible to break down under thermal constraints, and that therefore thermal stability of the color filter as a whole can be much improved by the choice of the cyan dye forming coupler. Examples of cyan-forming color couplers having a particularly good stability against light, heat and humidity are described in U.S. Pat. No. 4,342,825 and EP 0 269 766.
Since the dyes are formed in a coupling reaction between a color coupler and the color developing substance in its oxidized form, the structure of the color developing substance is decisive also for the dye-stability. In most embodiments of color development by means of color couplers p-phenylenediamine type developing agents are used. In published EP application 0 459 210 derivates of p-phenylenediamine yielding dyestuffs with improved fastness to light are described. Such color developing substances are therefore advantageously used in the production of color filters subjected lateron to radiation and/or thermal treatment.